This invention relates to a system for extracting information from a carrier wave and relates generally to the method and apparatus described in U.S. Pat. Nos. 4,106,007 and 4,218,655, the disclosures of which are incorporated herein by reference. As described in those patents, it is known that a modulation voltage can be superimposed on a power system voltage, at specified locations on the power system voltage such as a zero crossing, to cause wave shape perturbations in the carrier wave. In the embodiment described hereinafter, the carrier wave is the current wave of an electrical power distribution network.
Communication over electric power distribution lines is useful for signaling, meter reading, and load control, among other uses. However, communication over an electric distribution network is a complex undertaking. Each customer service constitutes a branch in the distribution feeder, and the branching is so extensive that it is impractical to provide filter and by-pass circuitry at each branch point. The distribution network is not an attractive medium for conventional communications due to the attenuation and dispersion of the signals and because noise levels tend to be high. To overcome the high noise levels, it is generally necessary to use narrow band filtering, error-detecting and error-correcting codes, and relatively high signal power levels at low bit rates.
The aforementioned problems arise in two areas. The first concerns transmitting information from the central source in the direction of energy flow to the individual customer premises. This transmission of information in the direction of energy flow is referred to as "outbound" signaling. Functions such as automatic meter reading and various alarm systems, however, require that information passes not only from a single source to the end user, but also from the end user back to the central station. This transmission of information in the direction opposite to that of the energy flow is referred to herein as "inbound" signaling.
In the system described in the aforementioned patents, each binary digit (a binary "1" or a binary "0") is composite. It is made up of four current pulse modulations located at preselected zero crossings of the electrical distribution network voltage waveform. These four current pulses are located within eight zero crossings (four complete cycles) of the waveform. The current pulse patterns for "1"s and "0"s are complementary.
Of course, any particular pulse pattern for a 1is not unique. By using different pulse patterns to define binary 1s and "0"s, it is possible to define a number of separate channels over which information can be transmitted in each eight half-cycle segment of the waveform. No matter what the channel, however, the pulse patterns for "1s" and "0s" are complementary.
It has been found, in studying inbound signals received over communications systems of the type described above, that pulse patterns are sometimes contaminated. As a result it is difficult at times to reconstruct the message originally sent, even with the relatively high signal levels and low bit rates of such systems.
To remedy the bit contamination problem, simple parity checking systems with error correction features have been tried, but they are not wholly satisfactory. Error correction in these cases has not always been completely accurate.
One example of the problem of bit contamination involves the bit detection scheme typically used in such systems. Heretofore, the detection scheme for such communications systems has generally involved summing the magnitudes of the current pulses detected. With such a scheme, a badly contaminated pulse can contaminate the entire bit even though the other pulses may not be contaminated.